Many airplanes are powered by one or more propellers driven by one or more intermittent combustion internal combustion engines, such as two-stroke or four-stroke internal combustion engines. In many such airplanes, during operation, the engine turns its corresponding propeller at a constant speed, which is commonly referred to as a constant-speed propeller. In order to address the changes in power requirements, the pitch of the propeller blades of the propeller is changed instead of changing the speed of the engine.
Each engine is connected to its corresponding propeller via a propeller drive system. The propeller drive system typically includes a plurality of gears arranged such that the propeller turns at a slower speed than a crankshaft of the engine.
Due to its mass and dimensions, the propeller has a high moment of inertia. Therefore, when the propeller turns during operation of the airplane, it tends to do so at a constant speed. On the other hand, the speed of rotation of the crankshaft of the engine varies. The speed of rotation of the crankshaft increases during power strokes of the engine and decreases during compression strokes of the engine. As a result, the portion of the propeller drive system that is connected to the propeller rotates at a constant speed, but the portion of the propeller drive system that is connected to the crankshaft of the engine varies in speed. This causes stress and wear of the gears in the propeller drive system and also causes noise and vibration.
Also, should resonance of the propeller drive system occur, the vibrations increase and the problems associated with these vibrations are exacerbated. In order to avoid resonance during most operating conditions, some propeller drive systems are designed with a stiffness that results in a resonance frequency that occurs at a speed of rotation that is below the idle speed of the engine. Therefore, during the normal operation range of the engine, which is at idle speed and higher, resonance of the propeller drive system should not occur. However, during engine start-up, the engine goes from rest to the idle speed of the engine, and as such at some point will turn at the speed that causes resonance of the propeller drive system. As such, during engine start-up, resonance of the propeller drive system occurs, which causes strong vibrations and may even cause the crankshaft to rotate backwards momentarily. As a result, the engine control unit that is responsible for the control of the fuel injection and ignition, among other things, may receive erroneous signals regarding the speed of rotation and position of the crankshaft, which could prevent successful engine start-up.
There is therefore a need for a propeller drive system that can dampen the torque peaks associated with the changes in speed of the crankshaft during operation of the engine and/or can avoid or reduce the impact of resonance of the propeller drive system during start-up of the engine.